IN an earlier communication1, discussing the analogy between the action of X-rays and of radiomimetic chemicals on thymonucleic acid, we commented on the nature of the changes which occur after the irradiation with X-rays has stopped. We pointed out that these changes might be due to the radicals produced by the irradiation, which themselves have a very short life, either initiating a chain reaction which results in the degradation of the nucleic acid, or forming intermediate products such as hydrogen peroxide or other peroxides which continue the action. In order to distinguish these possibilities, it was clearly desirable to determine the effect on the thymonucleic acid of hydroxyl radicals produced by alternative methods.